1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing a vitreous silica crucible, and in particular relates to a technique suitable for controlling the inner surface property of a vitreous silica crucible used for pulling a silicon single crystal.
2. Description of the Related Art
A silicon single crystal has been manufactured by the Czochralski method (the CZ method) using a vitreous silica crucible. In this method, polycrystalline silicon raw material is melted and retained in the vitreous silica crucible, a seed crystal of silicon single crystal is dipped into the silicon melt, and the seed crystal is gradually pulled while rotating it, to produce a silicon single crystal by use of the seed crystal as a core.
The vitreous silica crucible used in this method has two-layer structure including an outer layer containing a number of bubbles and a transparent inner layer. It is known that the property of the crucible inner surface which contacts silicon melt while pulling a single crystal influences the property of the obtained silicon single crystal, and thus influences the yield of silicon wafers which are the final products.
Thus, there is known a vitreous silica crucible having structure of an inner layer made of synthetic vitreous silica and an outer layer made of natural vitreous silica.
For example, when silicon is melted in a vitreous silica crucible and a single crystal is pulled therefrom, melt surface vibration occurs and thus seeding of a seed crystal becomes difficult. In this case, the melt surface vibration prevents pulling of a silicon single crystal or prevents single crystallization. The problem of the melt surface vibration becomes more eminent as the diameter of the silicon crystal increases. Thus, it has been further demanded to improve the inner surface property of the vitreous silica crucible. JP-A-2002-154894 discloses one method of coping with this problem.
Furthermore, in order to obtain a wafer having a diameter of 300 mm or more, i.e. approx. 450 mm, it has been demanded to increase the diameter of a silicon single crystal. This demand elongates the time for pulling a single crystal, and thus elongates the time during which the crucible inner surface contacts silicon melt of 1400 deg. C. or more. This causes the following problem.
When the time for pulling is elongated, the contact time of the crucible inner surface with silicon melt is also elongated. In this case, the crucible inner surface reacts with silicon melt, to cause crystallization in the surface or a shallow layer from the surface of the crucible inner surface. The reaction causes ring-shaped brown cristobalite (hereinafter, the ring-shaped cristobalite is referred to as “brown ring”). A cristobalite layer is not formed in the inside of the brown ring, or if any, the cristobalite layer is a thin layer. The brown ring increases the area as the increase of the operation time, and the adjacent brown rings merge and grow. Finally, the center of the brown ring is corroded to expose irregular vitreous silica corroded surface.
When tiny pieces of vitreous silica detaches from the vitreous silica corroded surface, dislocation is more likely to occur in the silicon single crystal, and thus deteriorates the single crystallization yield. In particular, in order to grow a silicon single crystal for manufacturing a wafer having a diameter of 300 mm, it is necessary to continue the operation of the CZ method for 100 hours or more, and thus the vitreous silica corroded surface is more likely to appear.
It is considered that the aforementioned brown ring is generated from a core which is a tiny scratch on the vitreous silica surface, a crystalline residual portion which is an unfused portion of material silica powder, or a defect of vitreous silica structure. It is considered that the number of the brown rings can be reduced by maintaining a good surface state of vitreous silica, or reducing the crystalline residual portion by fusing silica powder at higher temperature and for a longer time in the vitreous silica crucible manufacturing process. Furthermore, as described in JP-B-2811290 and JP-B-2933404, amorphous synthetic silica powder can be used as material silica powder for forming the inner surface.
Synthetic vitreous silica made of amorphous synthetic silica powder contains impurities in an extremely small amount, and thus the use of synthetic silica powder can reduce the number of brown rings. However, the crucible having an inner layer of synthetic vitreous silica has a drawback in that melt surface vibration is more likely to occur when polysilicon is melted in the crucible having an inner layer of synthetic vitreous silica than a crucible having an inner layer made of natural vitreous silica. The vibration is in particular observed from a seeding process to a shoulder formation process, and at initial stage of pulling a first half of the body of a single crystal. Therefore, the melt surface vibration necessitated longer time for seeding, disturbed crystallization, and necessitated meltback, which led to drop in the productivity.
Furthermore, JP-A-2001-89171 discloses, in the embodiment, an apparatus for manufacturing a vitreous silica crucible by the rotating mold method carried out by arc fusing. This document describes that the apparatus is provided with a means to prevent mix-in of carbon, and thus it is possible to manufacturing a vitreous silica crucible having an inner surface with reduced impurities. Furthermore, JP-B-3926167 discloses, in the embodiment, an apparatus for manufacturing a vitreous silica crucible, which is provided with an are fusing apparatus having a radiation thermometer disposed above the arc electrodes.